The invention relates to actuators. Actuators embodying the invention and to be described in more detail below incorporate an electric motor which is arranged to produce translational movement of an output member. The actuators to be described may be used as part of an automatic vehicle speed control system for controlling a vehicle to run at a desired speed. In such systems, a control signal is developed depending on any error between actual and desired speeds for the vehicle, and this control signal energises the actuator whose output member adjusts the engine power (such as by adjusting the engine throttle in the case of an internal combustion engine) so as to bring the vehicle to the desired speed. However, the actuators to the described are not limited to such use.